In the past, various laminated windshields (windows) have been provided for aircraft. Such laminations can be made of rigid layers of glass or plastic or combinations of glass and plastic sandwiched together by means of interlayers such as urethane, polyvinyl butyral or silicone.
Commonly the windshields are provided with electrical systems or circuits that are bonded between the layers and that heat the windshield when energized.
One type of electrical system provided in the past for heating includes a thin film of conductive metal or metal oxide adhered on the inner surface of a rigid outer windshield layer or face ply.
There are disadvantages, however, to using such conductive films. For example, such films can be less transparent than desired, thus impairing vision. The films have been found to be particularly undesirable for use with low voltage systems, e.g., 28 volt systems found in small aircraft and helicopters. For example, if the films are made thick enough so that they conduct enough current for proper heating, they are far less transparent than desired.
Another type of electrical system used in such windows/ windshields for heating is based on using a circuit of wires embedded between layers in the laminate. Generally the wires are hot pressed below the surface of the interlayer adjacent the face ply. Since it is desired not to impair vision through the windshield, fine wires, e.g., wires having a diameter of 0.5 to 1.5 mils, are used and the wires are spaced apart in a geometric pattern.
There are, however, disadvantages to using such small diameter wire. For example, such wires break easily during the manufacturing process causing process inefficiencies and rejects. Also images viewed through the heater zone of a typical wire heated windshield can appear blurred and distorted. Such distortion is caused by the thermal gradient created in the interlayer around and between wires which, in turn, results in gradients in indices of refraction. Such thermal gradients are the result of the wire having, at most, only tangential contact with the face ply being heated while being surrounded by interlayer material which is a poor heat conductor. When the heater is energized the interlayer becomes very hot at the wire surface which causes the steep thermal gradient and resulting optical distortion.
Another approach to providing a heater circuit on the surface of a glass panel in a window laminate is disclosed by Coale, et al. in U.S. Pat. No. 2,932,710. The electrical conductors disclosed by Coale, et al. are applied to the glass surface by first masking the surface completely with a protective film. The pattern of the electrical conductors is then formed through the film by scribing or using photographic techniques to thereby expose only that portion of the glass that forms the desired conductor pattern. A metal such as silver or copper or gold is then applied by "conventional mirror-producinq methods" or by vaporization to the exposed pattern on the glass surface. The protective film is then removed from remaining portions of the glass leaving the conductive metal lines on the glass surface.
When a circuit that comprises electroconductive lines is provided on a glass surface for heating a windshield that incorporates the circuit, it is important that the lines adhere to the surface. For example, if such lines do not adhere, non-uniform heating can result which can distort vision through the windshield. Such vision distortion can be intolerable, for example, in aircraft windshields.
Although the disclosure of Coale, et al. relates to forming electroconductive lines on glass, aircraft windshields onto which such lines can be formed for use as a heater may be made entirely of plastic as a weight-saving measure.
There is a need in the art, therefore, for a simple and economical process for forming electroconductive lines comprising thin films of metal onto the surface of a glass and/or a plastic substrate where adherence of the lines to the substrate is enhanced.